1. Field of the Invention
The present invention relates to a signal modulation circuit, and particularly relates to a circuit configured to perform delta-sigma modulation.
2. Description of the Related Art
Typically, delta-sigma modulation (ΔΣ modulation) is employed at, e.g., a switching amplifier. A delta-sigma modulator includes a subtracter, an integrator, a quantizer, and a quantization error feedback circuit. The subtracter is configured to calculate the difference between an input signal and a quantized feedback signal. The integrator is configured to integrate a differential signal. The integrated signal is quantized by the quantizer, and is output as a signal of one bit=two values, for example.
JP-A-2007-312258 discloses a delta-sigma modulation circuit including an integrator group, an adder group, a quantizer, and a pulse width revaluation circuit. JP-A-2007-312258 also discloses that after conversion into a one-bit signal synchronized with a sampling clock, the one-bit signal is output. JP-A-2007-312258 further discloses that a D flip-flop is used as the quantizer. Moreover, JP-T-2012-527187 also discloses a delta-sigma modulation circuit.
Japanese Patent No. 3369503 describes that the voltage of a switching signal obtained by pulse amplification of a quantized output signal is reduced by resistive division on a feedback loop for feeding back the switching signal to a delta-sigma modulator, thereby generating a feedback signal. Moreover, in FIG. 8 or 9 of Japanese Patent No. 3625169, it is illustrated that a power amplifier stage positive/negative output waveform of a differential output is directly fed back without resistive division.
In order to feed back, as it is, deformation of the waveform of the switching signal due to, e.g., ripple contained in constant voltage applied to a power amplifier or external noise, it is advantageous that the voltage of the switching signal is reduced by resistive division to generate the feedback signal on the feedback loop for feeding back the switching signal to the delta-sigma modulator. On the other hand, another disadvantage might be caused.
That is, in the case where a resistance value is low in resistive division, if the speaker output voltage driven by the switching signal is high, a component (s) is easily damaged due to a high power consumption and heat generation. This leads to a component size increase. Conversely, when the resistance value is high, there is a disadvantage of lowering performance due to a switching speed decrease. In particular, a printed circuit board pattern on the feedback loop tends to be relatively long as compared to another line. Due to this tendency, attenuation of a high-frequency signal on the feedback loop cannot be ignored, and therefore, the performance is easily lowered.
When the method for directly feeding back the power amplifier stage positive/negative output waveform of the differential output without resistive division is employed, a resistor to ground is not required, and therefore, no current flows through the ground GND. On the other hand, the signal is not attenuated by resistive division. For this reason, there is a disadvantage that a switching waveform of rail-to-rail supply voltage is dragged in a certain pattern, and as a result, noise is easily caused. Moreover, variation in resistance value between two positive and negative feedback paths causes non-linear secondary distortion in delta-sigma modulation. This leads to a worse performance.